1. Field of the Invention
This invention relates to the production of coating compositions comprising a liquid radiation curable suspending medium and having capsular material dispersed therein. In particular, it is concerned with a process of making liquid radiation curable coating compositions containing microcapsules starting with volatile solvent microcapsular dispersions. The liquid radiation curable coating compositions so produced are particularly useful, for example, for the production of pressure-sensitive carbonless copy papers.
2. Prior Art
Carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of a paper substrate is coated with what is referred to as CB coating, the CB coating containing one or more color precursors generally in capsular, and more preferably microcapsular, form. At the same time, the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating which contains one or more color developers. Both the color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form. This is true until the CB and CF coatings are brought into abutting relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the color precursor. At this time the color precursor transfers to the CF coating and reacts with the color developer therein to form an image. Carbonless paper has proved to be an exceptionally valuable image transfer medium for a variety of reasons only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and the CF are in an inactive state as the co-reactive elements are not in contact with one another. Patents relating to carbonless paper products are:
U.S. Pat. No. 2,712,507 (1955) to Green
U.S. Pat. No. 2,730,456 (1956) to Green et al
U.S. Pat. No. 3,455,721 (1969) to Phillips et al
U.S. Pat. No. 3,466,184 (1969) to Bowler et al
U.S. Pat. No. 3,672,935 (1972) to Miller et al
A third generation product which is in an advanced stage of development and commercialization at this time and which is available in some business sectors is referred to as self-contained paper. Very generally stated self-contained paper refers to an imaging system wherein only one side of the paper needs to be coated and the one coating contains both the color precursor, generally in encapsulated form, and the color developer. Thus when pressure is applied, again as by a typewriter or other writing instrument, the color precursor capsule is ruptured and reacts with the surrounding color developer to form an image. Both the carbonless paper image transfer system and the self-contained system have been the subject of a great deal of patent activity. A typical autogeneous record material system, earlier sometimes referred to as "self-contained" because all elements for making a mark are in a single sheet, is disclosed in U.S. Pat. No. 2,730,457 (1956) to Green.
A disadvantage of coated paper products such as carbonless and self-contained stems from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacturing process. In the application of CB coatings, the coating composition generally comprises a dispersion of microcapsules in an aqueous medium containing also a binder for the microcapsules. This aqueous coating composition requires removal of the excess water by drying, which drying entails the use of complex and expensive equipment and a high input of heat energy to continuously dry a substrate coated with an aqueous coating composition.
The application of heat not only is expensive, making the total product manufacturing operation less cost effective, but also is potentially damaging to the color forming ingredients which are generally coated onto the paper substrate during manufacture. High degrees of temperature in the drying step require specific formulation of wall-forming compounds which permit the use of excess heat. The problems encountered in the actual coating step are generally attributable to the necessity for a heated drying step following the coating operation.
Many of the particular advantages of the process of this invention are derived from the fact that a radiation curable coating composition is produced which may be used to coat the paper substrate. This is in contrast to the coating used by the prior art which have generally required an aqueous or organic solvent coating. For purposes of this application the term "100% solids coating" will sometimes be used to describe the coating operation and should be understood to refer to the fact that a radiation curable coating composition is used and therefore the drying step usually present in the manufacture of paper and in coating has been eliminated.
In general, patents concerned with the production and application of liquid resin compositions containing no volatile solvent, which resin compositions are subsequently cured by radiation to a solid film are:
U.S. Pat. No. 3,551,235 (1970) to Bassemir et al
U.S. Pat. No. 3,551,246 (1970) to Bassemir et al
U.S. Pat. No. 3,551,311 (1970) to Nass et al
U.S. Pat. No. 3,558,387 (1971) to Bassemir et al
U.S. Pat. No. 3,661,614 (1972) to Bassemir et al
U.S. Pat. No. 3,720,534 (1973) to Macanlay et al
U.S. Pat. No. 3,754,966 (1973) to Newman et al
U.S. Pat. No. 3,772,062 (1973) to Shur et al
U.S. Pat. No. 3,772,171 (1973) to Savageau et al
U.S. Pat. No. 3,801,329 (1974) to Sandner et al
U.S. Pat. No. 3,819,496 (1974) to Roskott et al
U.S. Pat. No. 3,847,769 (1974) to Garratt et al
U.S. Pat. No. 3,847,768 (1974) to Kagiya et al
These compositions generally also contain a pigment or a dye. Such resin compositions are useful for protective coatings and fast drying inks. U.S. Pat. Nos. 3,754,966 and 3,720,534 describe the production of an ink releasing dry transfer element which can be used as a carbon paper or typewriter ribbon. The above mentioned patents do not disclose the use of microcapsules in their liquid radiation curable compositions and therefore are not faced with the problem solved by the instant invention, that of converting a dispersion of microcapsules in volatile solvent to a dispersion of microcapsules in a radiation curable coating composition.
The use of microcapsules in coating compositions which are cured by radiation to produce transfer (CB) sheets is disclosed in commonly-assigned co-pending application U.S. Application Ser. No. 684,462 filed May 7, 1976, now U.S. Pat. No. 4,091,122. In this co-pending application, the microcapsules contained a chromogenic material (color precusor) in solution in an oil. The CB coating compositions used therein were prepared by dispersing a dry powder of the microcapsules in a radiation curable substance. The cured radiation curable substance acted as binder for the microcapsules in the finished transfer sheet. Preparation of the dry powder of microcapsules was by spray drying an aqueous dispersion of microcapsules.
In the present invention, the separate steps of drying and redispersing the microcapsules in a radiation curable binder can be eliminated. Dispersions of microcapsules in any volatile solvent can be converted to dispersions of microcapsules in a radiation curable coating composition by applying heat and vacuum to a mixture of the microcapsule dispersion in the volatile solvent and a radiation curable suspending medium. Sufficient heat is needed only to vaporize the solvent. Temperatures may be maintained at room temperature or slightly higher. Damage due to storage and handling of a microcapsular powder are substantially eliminated. The use of heat in combination with a vacuum permits the effective removal of the volatile solvent without requiring an excess of either heat or vacuum. The process can be operated continuously. For purposes of this application the term volatile solvent shall be used to refer to aqueous and non-aqueous low boiling solvents and would include among others water and organic solvents.